In the field of positron emission tomography (PET), it is known that to measure the energy absorbed from a gamma ray interacting in a scintillation crystal, the light from a crystal may be determined by integrating the photo sensor current. This current signal represents the amount of light collected by the sensing photomultiplier tubes (PMTs) or photodiodes. As graphically illustrated in FIG. 1, the integration may be performed by using a uniformly weighted summation of digital samples of the signal. In this method the level of the signal at time t(0) is zero volts.
Alternating current (AC) capacitive coupling strategies are commonly used in (PMT) based (PET) data acquisition (DAQ) signal paths, particularly for detectors that have a positive high voltage. AC capacitive coupling strategies have also been applied in most of the avalanche photodiode (APD) and other solid-state detectors, such as silicon photomultiplier (SiPM). Baseline shift (i.e., the level at time t(0) deviating from zero) resulting from count-rate variant is an intrinsic artifact in an AC coupling signal path, since the direct current (DC) component of the signal is blocked while the AC components are passed on, which results in the total charge integral across the coupling capacitor remaining zero.
FIG. 2 shows a typical PMT 201 high voltage (HV) bleeder network or bias network with positive HV 203 and AC coupled anode (A) and last dynode (Dy) outputs. As indicated, high voltage capacitors C1 and C2 are required to couple the high voltage potential anode (A) or dynode (Dy) signals to subsequent readout circuits. Similarly in FIG. 3, which illustrates an APD front-end readout circuit with charge sensitive preamplifier readout, an HV capacitor is used to couple an APD signal with HV potential to the following charge-sensitive preamplifier. An AC-coupling capacitor blocks or removes the DC component of the signals such that only AC components pass through. This has the disadvantages of degrading signal low frequency components, due to the high-pass CR filter formed from each coupling capacitor combined with the impedance of the following circuitry. Moreover, as indicated above, AC capacitive coupling causes baseline shift or baseline wander in count-rate variant conditions.
According to the Campbell theorem, an average DC component VDC of a series of pulses is VDC=νARV, where ν is the average pulse count-rate, AR is the area of a pulse having unity amplitude, and V is the average amplitude of the signal pulse. Adverting to FIGS. 2 and 3, since signal pulses A and Dy in FIG. 2 and APD output in FIG. 3 are unipolar, VDC is not zero. Further, VDC will be varied when count-rate fluctuates, as VDC is proportional to count-rate ν. The transmitted signals through the coupling capacitor will be DC-balanced, i.e., the total integral of positive and negative areas is zero. Since VDC will shift to zero after AC coupling, the baseline of the original signals will accordingly shift. This creates errors in timing and energy acquisitions if the baseline is not corrected and restored constantly.
Baseline restoration (BLR) has been of interest in related high energy physics fields for decades. Both analog and digital solutions, such as digital BLR, have been implemented in PET DAQ systems. However, digital BLR is challenging for systems in which the detector output signal includes a significant amount of noise. For example, APD signals have significantly higher noise floors mainly due to intrinsic APD excess noise.
The fundamental analog BLR circuit is a Robinson baseline restorer, illustrated in FIG. 4. The gain (xG) stage 401 is necessary to buffer capacitor CC along with diode D1, thereby forming CD baseline circuitry.
FIG. 5 shows an improved version of the “quasi-ideal” BLR circuit. For an input pulse with negative polarity, diode D1 will be turned off when point “A” is below zero potential. Diode D1 will turn on when the input pulse rises above zero, then RA, D1, and amplifier 501 form a closed loop forcing point “A” to ground or zero potential. As is evident from FIGS. 4 and 5, prior art BLR circuits require a capacitor, a diode, and an amplifier as the main components for forcing the baselines to maintain a zero potential. Such circuits require low voltage (e.g., about ±5V or less) DC power supplies to support the operational amplifier. However, for PMT based PET detectors, in many cases no low voltage DC power supply is required, and, thus, it is unavailable. Although a high voltage (e.g., between about 1000 V and about 2000 V) is required for such detectors, the high voltage cannot be used for supplying the OPAMP.
A need therefore exists for an improved BLR circuit for an AC capacitive coupling signal path, with only passive components, eliminating the operational amplifier and DC power supplies to support the operational amplifier, to achieve more accurate timing, higher energy resolution, and lower power consumption for PET data acquisition systems.